Generally, a magnetic layer of a magnetic recording medium is prepared by coating on a non-magnetic support, such as a polyester film, a coating solution containing magnetic particles and inorganic particles such as carbon black particles or alumina particles, providing a magnetic orientation treatment and then drying the coating solution. The inorganic particles are added to improve antistatic property, wear resistance and running durability, and are uniformly dispersed in a binder and an organic solvent with the magnetic particles to prepare the coating solution.
Recently, as magnetic recording media have been used in various fields, various characteristics have been demanded for the media and such demand has become very severe. Particularly, there has been a need for a magnetic recording medium which has excellent durability and which is suitable for higher density recording.
On the one hand, in order for a magnetic recording medium to achieve high density recording, certain conditions must be satisfied. First, inorganic particles, such as the magnetic particles as a main component in a magnetic coating composition should be uniformly dispersed in a binder to form the coating solution. Second, excellent magnetic characteristics, extremely low surface roughness and extremely smooth coated surface should be obtained after the coating solution is coated and dried.
On the other hand, it is known that abrasive agent particles such as .alpha.-Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3 or .alpha.-Fe.sub.2 O.sub.3 are added into the magnetic layer to improve sliding properties of a magnetic head and to improve durability of the magnetic recording medium. It is also known that lubricating agent particles such as carbon black, graphite or molybdenum sulfide are added into the magnetic layer to improve running properties. However, the above described abrasive agent particles and lubricating agent particles are non-magnetic particles and when they are used in a large amount, the electromagnetic properties deteriorate. Therefore, various techniques regarding shapes, sizes and the mixing ratios have been developed to attempt to overcome these problems.
Such techniques are specifically disclosed in Japanese Patent Publication Nos. 28642/77, 18561/77 and 9041/79 and Japanese Patent Application (OPI) Nos. 179945/82, 189826/83, 6439/82, 88307/77 and 119932/81. The term "OPI" as used herein means an unexamined published Japanese Patent Application.
However, the above techniques are not satisfactory and there are still some problems which remain to be solved. Thus in the known techniques, the above described lubricating agent particles and abrasive agent particles are dispersed with magnetic particles in a magnetic coating solution and are uniformly distributed in a magnetic layer.
Since the above described lubricating agent particles and abrasive agent particles are uniformly distributed in the magnetic layer, most of particles do not effectively work their functions because they are not near the surface of the magnetic layer and thus do not contact the magnetic head, guide parts of a VTR (Video Tape Recorder) and the system parts of a cassette. To achieve sufficient improvement in durability and running properties, the abrasive agent particles and lubricating agent particles need to be added in a large amount. However, when they are added in a large amount to provide sufficient durability and running properties on the surface of the magnetic layer, the electromagnetic properties deteriorate and the number of drop outs increases due to the increased contact of the particles with the magnetic head and guide parts of the VTR and with the system parts of a cassette. On the other hand, when they are added in a smaller amount, sufficient durability and running properties can not be realized.